How endangered is sexual reproduction of high-mountain plants by summer frosts? Frost resistance, frequency of frost events and risk assessment

In temperate-zone mountains, summer frosts usually occur during unpredictable cold spells with snow-falls. Earlier studies have shown that vegetative aboveground organs of most high-mountain plants tolerate extracellular ice in the active state. However, little is known about the impact of frost on reproductive development and reproductive success. In common plant species from the European Alps (Cerastium uniflorum, Loiseleuria procumbens, Ranunculus glacialis, Rhododendron ferrugineum, Saxifraga bryoides, S. moschata, S. caesia), differing in growth form, altitudinal distribution and phenology, frost resistance of reproductive and vegetative shoots was assessed in different reproductive stages. Intact plants were exposed to simulated night frosts between ?2 and ?14 °C in temperature-controlled freezers. Nucleation temperatures, freezing damage and subsequent reproductive success (fruit and seed set, seed germination) were determined. During all reproductive stages, reproductive shoots were significantly less frost resistant than vegetative shoots (mean difference for LT₅₀ ?4.2 ± 2.7 K). In most species, reproductive shoots were ice tolerant before bolting and during fruiting (mean LT₅₀ ?7 and ?5.7 °C), but were ice sensitive during bolting and anthesis (mean LT₅₀ around ?4 °C). Only R. glacialis remained ice tolerant during all reproductive stages. Frost injury in reproductive shoots usually led to full fruit loss. Reproductive success of frost-treated but undamaged shoots did not differ significantly from control values. Assessing the frost damage risk on the basis of summer frost frequency and frost resistance shows that, in the alpine zone, low-statured species are rarely endangered as long as they are protected by snow. The situation is different in the subnival and nival zone, where frost-sensitive reproductive shoots may become frost damaged even when covered by snow. Unprotected individuals are at high risk of suffering from frost damage, particularly at higher elevations. It appears that ice tolerance in reproductive structures is an advantage but not an absolute precondition for colonizing high altitudes with frequent frost events.     

Summer frost resistance and freezing patterns measured in situ in leaves of major alpine plant growth forms in relation to their upper distribution boundary

Summer frost resistance and ice nucleation temperatures for 33 alpine plant species were measured in situ to avoid the shortcomings of laboratory tests. Species were selected to investigate the relationship between plant stature and upper distribution boundary, and frost resistance and freezing patterns. The species tested in situ were on average 1.1 K (± 0.2, SE) frost hardier than in laboratory tests. Frost resistance (LT₅₀) ranged from ?4.5 to ?14.6 °C and appeared insufficient to protect against air temperature minima, corroborating reports of natural frost damage. All species tolerated extracellular ice formation (recorded at ?1.9 ± 0.2 °C; E1). Initial frost damage occurred at average temperatures 4.9 K below E1. In 64% of the species a second exotherm (E2) and frost damage were recorded between ?3.7 and ?9.4 °C. In the highest ranging species E2 was not detectable. Frost resistance increased with increasing upper distribution boundary (0.4 K per 100 m), corresponding well with the altitudinal decrease in air temperature minima. No relationship between plant stature and frost resistance was found. Graminoids were significantly frost hardier than other growth forms. Frost survival at high altitudes will depend not only on altitudinal increase in frost resistance but also on freezing avoidance strategies, snow cover protection and a high recuperation capacity.     

Effect of frost nights and day and night temperature during dormancy induction on frost hardiness, tolerance to cold storage and bud burst in seedlings of Norway spruce

For trees, the ability to obtain and maintain sufficient levels of frost hardiness in late autumn, winter and spring is crucial. We report that temperatures during dormancy induction influence bud set, frost hardiness, tolerance to cold storage, timing of bud burst and spring frost hardiness in seedlings of Norway spruce (Picea abies (L.) Karst.). Bud set occurred later in 12°C than in 21°C, and later in cool nights (7°C) than in constant temperature. One weekly frost night (−2.5°C) improved frost hardiness. Cool nights reduced frost hardiness early, but improved hardiness later during cold acclimation. Buds and stems were slightly hardier in 21°C than in 12°C, while needles were clearly hardier in 12°C. Cold daytime temperature, cool nights and one weekly frost night improved cold storability (0.7°C). Seedlings receiving high daytime temperatures burst buds later, and were less injured by light frost some days after bud burst.     

An investigation of the frost hardiness of grapevine (Vitis vinifera) during bud break

The aim of this investigation was to assess ice nucleation and frost resistance of two varieties of grapevine (Siegrebbe and Madeleine Angevine) during bud burst under radiative freezing conditions analogous to those during Spring in the UK. During bud burst, grapevines were almost entirely resistant to freezing during frosts of less than -3°C by virtue of their ability to supercool. The risk of frost damage increased significantly as bud development progressed, and once buds had passed growth stage DS3 they became more sensitive to freezing and freezing damage was more extensive. The two varieties did not differ in frost resistance but, because of its earlier developing habit, variety Siegrebbe could be more prone to frost damage in the field. Buds were more prone to damage after freezing once bud burst had commenced and the damage could not be reversed by acclimating plants for periods of 7 to 21 days at 4°C in an 8 h photoperiod. Such acclimation appeared to predispose frozen buds to more extensive damage.     

Frost Injury as a Possible Inciting Factor in Bud and Shoot Necroses of Fraxinus excelsior L.

Large numbers of European ash have died in Poland in all age classes during the last ten years. The characteristic symptom occurring on shoots of planted and self‐sown seedlings was bark necroses starting from the shoot apex, necrotic buds, or leaf and twig scars. The results showed that in the bud tissue of cold acclimated European ash extracellular and intracellular ice formation occurred at approximately ?9 and ?32°C, respectively. In deacclimated plants in spring water supercooling is limited by the heterogenous ice nucleation temperature and consequently the cold tolerance is ?9 to ?4°C for bud tissues and ?13 to ?9°C for shoots. Isolations of fungi were performed from dead buds and from necroses occurring on the main stem. Alternaria alternata, Fusarium lateritium and Phomopsis scobina were among the fungi occurring in both these organs at frequencies of more than 7%. Cylindrocarpon heteronemum, Diplodia mutila and Tubercularia vulgaris from necroses were only isolated in frequencies; 3.3, 1.2 and 5.4%, respectively. It seems likely that freezing injury is the inciting factor, which combined with fungal colonization manifests itself as fatal damage to European ash buds and shoots.     

Seasonal Fluctuations of Ascorbate-Related Enzymes: Acute and Delayed Effects of Late Frost in Spring on Antioxidative Systems in Needles of Norway Spruce (Picea abies L.)

Seasonal changes of ascorbate peroxidase and monodehydroascorbateradical reductase activities were studied in foliar tissuesof Norway spruce (Picea abies L.). In mature needles, APX activitiesdid not show seasonal fluctuations and were similar to thosefound in resting buds. Monodehydroascorbate radical reductaseactivity was higher in needles than in buds and higher in winterthan in summer. Maximum activities of both enzymes were foundbefore bud break and minimum activities in newly formed needles.When spruce seedlings were exposed to an artifical frost eventof –5°C for one night in spring, ascorbate peroxidaseactivity declined in young needles before the onset of visibleinjury but corresponding to a sudden upsurge in lipid peroxidation.After one week, some shoots showed severe symptoms of injury,some were slightly injured and others did not show any visibleinjury. In lethally injured needles, antioxidative protection(ascorbate peroxidase, monodehydroascorbate radical reductase,glutathione reductase, glutathione, ascorbate, superoxide dismutase)had collapsed. Surviving needles showed a coordinated increasein all components of the antioxidative system suggesting anefficient induction of defense systems. However, enhanced protectionwas observed only transiently. In fall, needles that had beenexposed to frost in spring contained significantly less antioxidantsthan unstressed needles indicating that unseasonal frost causedmemory effects. (Received September 16, 1995; Accepted May 28, 1996)     

Damage in needle tissues after infection with Chrysomyxa rhododendri increases cuticular conductance of Picea abies in winter

Chrysomyxa rhododendri is a rust which infects Picea abies growing near the alpine timberline. Attacked needles are normally shed, but few remain on shoots. We hypothesised that these needles increase transpiration of Picea during winter. Partly damaged, completely damaged and healthy needles of an infected tree as well as healthy needles of a resistant tree were compared in a microscopy analysis, and needle conductance of shoots was measured gravimetrically. Despite needle shedding, more than 6% of needles remaining on infected tree shoots were damaged. Partly damaged needles showed local brownish areas in the periphery and completely damaged needles necrotic parenchyma and epidermal tissues. Cuticular conductance of affected shoots was up to 25.23?±?2.75 mmol m^?2 s^?1 at moderate water potential and thus twofold higher than in the resistant tree. Needle shedding reduces negative effects of Chrysomyxa infections during summer, but remaining damaged needles impair tree water relations in winter.     

Predicting spring frost sensitivity by bud development and temperature sum in Norway spruce seedlings

Nurseries would benefit greatly if frost hardiness (FH) of seedlings could be predicted by some environmental variable or by bud development in spring. We investigated the FH of 1-year-old Norway spruce (Picea abies (L.) Karst.) seedlings of local origin. The seedlings were stored frozen until incubated in the growth chamber at six different temperature sums (TSs) (0, 55, 88, 142, 185 and 240 d.d., >5°C) from mid-February to mid-March. FH of the buds, stems and previous year needles was assessed on three occasions. When the TS was 88 d.d. or less, buds exhibited only microscopic signs of development, even when seedlings tolerated temperatures below −10°C. As TS increased, primordial needles and primordial stems of buds grew while FH weakened, especially in previous year needles. When the TS was at least 142 d.d., all plant parts were frost hardy to approximately −6°C. Monitoring TS and bud development can help predict FH of Norway spruce seedlings in spring. However, more studies with seedlings of different ages and from multiple locations are necessary to appreciate the generality of our results.     

Freezing and high temperature thresholds of photosystem 2 compared to ice nucleation, frost and heat damage in evergreen subalpine plants

Freezing and high temperature thresholds of photosystem 2 (PS2), ice formation and frost and heat damage were measured in leaves of evergreen subalpine plants under conditions of naturally low (winter) to high (summer) PS2 efficiencies (F_V/F_M). The temperature‐dependent change in basic Chl fluorescence (F₀) (T‐F₀) technique that is usually used to assess the high temperature threshold of PS2 in a new approach was applied to test freezing temperature thresholds of PS2. T‐F₀ curves (+5 °C to ?10 °C at 2 K h^?1) revealed a significant, sudden increase in F₀ on extracellular ice formation (?4.0 or ?5.5 °C). The rise in F₀ was recorded 0.3–0.6 K below ice nucleation (10–20 min later) and was produced by freeze dehydration of cells. The rise in F₀ was not caused by frost damage, as during winter LT₅₀ was lower than ?27 °C and not by formation of ice on the leaf surface. Hence, F₀ measurements during freezing are a useful tool to distinguish between surface ice and extracellular ice inside the leaf tissue which cannot be differentiated by other ice‐detecting methods. PS2 efficiency significantly affected the shape of the high temperature T‐F₀ curves (20–65 °C at 1 K min^?1). Under F_V/F_M >0.6, two F₀ maxima were recorded. The fast rise phase to the first F₀ maximum corresponded with tissue heat damage (LT₅₀: 46.9–54.3 °C). The second F₀ maximum occurred at leaf temperatures between 55 and 60 °C. Under F_V/F_M <0.2 only, the second F₀ maximum was detectable. Lack of awareness of the missing F₀ maximum would lead to an overestimation of the PS2 high temperature threshold by >10 K; hence, under low F_V/F_M, it cannot be determined by the T‐F₀ technique.     

Abscisic acid content at defined levels of bud dormancy and frost tolerance in two contrasting populations of Picea abies grown in a phytotron

Seedlings of a southern (Romanian) and a northern (Swedish) population of Picea abies were cultivated under continuous light and 20°C for 10 weeks. To arrest growth, induce terminal bud dormancy and promote frost tolerance the seedlings were then exposed to 16 h nights for 12 weeks, with gradually lower temperature during the last 6 weeks. Samples for estimating the abscisic acid content of the needles were taken just before the onset of the night treatment, at day 3 of the treatment, and then with one, and later 2 week, intervals. From the second week onwards (third week for frost tolerance) bud dormancy and frost tolerance were assessed at the same time as abscisic acid (ABA) determinations. Phosphate-buffered saline extracts were purified on mini-columns (in some cases immunoaffinity colums) and quantified by HPLC. The degree of dormancy was estimated by transferring the seedlings to growth conditions and determining the number of days until growth was resumed. The frost tolerance of the needles exposed to –10°C and –20°C was classified in 6 classes. The frost tolerance of the terminal buds was estimated as the number of seedlings that showed some growth after 6 weeks in growth conditions. The night treatment rapidly induced terminal bud dormancy in both populations, but the release of dormancy occurred earlier in the northern population. The needles and the terminal buds became highly frost tolerant more rapidly in the northern than in the southern population and before the temperature decrease. The degree of dormancy began to decline before full frost tolerance was obtained in the southern population and this decline continued in both populations, while frost tolerance remained at a high level. The southern population showed a transient peak in ABA content at day 3. Although the ABA content of the northern population was lower than in the southern before the 16-h night treatment, it increased in the northern population during the treatment period, in particular after the temperature decrease.     

Deep supercooling enabled by surface impregnation with lipophilic substances explains the survival of overwintering buds at extreme freezing

The frost survival mechanism of vegetative buds of angiosperms was suggested to be extracellular freezing causing dehydration, elevated osmotic potential to prevent freezing. However, extreme dehydration would be needed to avoid freezing at the temperatures down to ?45°C encountered by many trees. Buds of Alnus alnobetula, in common with other frost hardy angiosperms, excrete a lipophilic substance, whose functional role remains unclear. Freezing of buds was studied by infrared thermography, psychrometry, and cryomicroscopy. Buds of A. alnobetula did not survive by extracellular ice tolerance but by deep supercooling, down to ?45°C. An internal ice barrier prevented ice penetration from the frozen stem into the bud. Cryomicroscopy revealed a new freezing mechanism. Until now, supercooled buds lost water towards ice masses that form in the subtending stem and/or bud scales. In A. alnobetula, ice forms harmlessly inside the bud between the supercooled leaves. This would immediately trigger intracellular freezing and kill the supercooled bud in other species. In A. alnobetula, lipophilic substances (triterpenoids and flavonoid aglycones) impregnate the surface of bud leaves. These prevent extrinsic ice nucleation so allowing supercooling. This suggests a means to protect forestry and agricultural crops from extrinsic ice nucleation allowing transient supercooling during night frosts.     

Genotypic variation in cold tolerance influences the yield of Miscanthus

When grown in Europe, Miscanthus genotypes often produce yields lower than their potential due to late emergence of shoots in the spring or to damage from late frosts when shoots emerge too early. Here, we investigate genotypic variation in the base temperature (T_b) for shoot emergence and in the lethal temperature for shoots (LT₅₀) in four Miscanthus genotypes. In all genotypes, lowering temperature increased the time to shoot emergence, with T_b ranging from 8.6°C in Sac‐5 to 6°C in Sin‐H9. Frost treatments below ?8°C resulted in a marked reduction in growth in all four genotypes. Sin‐H9 was the most frost tolerant with an LT₅₀ of ?9.3°C. There was little variation found in leaf osmotic potential, but leaf moisture content was significantly lower in Sin‐H9 than in the other genotypes. The lower thermal requirement for emergence and lower LT₅₀ seen in Sin‐H9 was incorporated into a model of Miscanthus production. The model showed an extended growing season that was predicted to increase yields by up to 25%.     

The Influence of Photoperiod and Temperature on the Development of Frost Hardiness in Seedlings of Pinus silvestris and Picea abies

The influence of short days and low temperature on the development of frost hardiness in seedlings of Scots pine (Pinus silvestris L.) and Norway spruce [Picea abies (L.) Karst.], grown for 6 months in glasshouses and climate chambers, was investigated. The degree of hardiness was estimated by freezing the shoots of the seedlings to predetermined temperatures. After 8 weeks in a glasshouse the viability of the seedlings was determined by establishing bud flushing. The most effective climate for the development of frost hardiness was short days (SD) and low temperature (2°C); the next most effective was SD and room temperature (20°C). However, long days (LD) and low temperature also had a marked effect on the development of hardiness. A combination of 3 weeks’treatment with SD and 20°C, and 3 weeks with SD and 2°C gave the same results as 6 weeks with SD and 2°C. The results clearly demonstrate the importance of the photoperiod prior to low temperature for the development of frost hardiness. In conclusion both short days and low temperature induce frost hardiness development. Probably this occurs by initiation of different processes in the two cases. The degree of frost hardiness development appears to depend on the sum of these different processes and on the timing between them.     

Influence of frost on nutrient resorption during leaf senescence in a mangrove at its latitudinal limit of distribution

The latitudinal distribution of mangrove species is limited mainly by low temperature. Leaf scorch and massive leaf fall are the predominant symptoms of frost damage. Nutrient resorption during leaf senescence is an important adaptation mechanism of mangroves. Abnormal defoliation disturbs nutrient resorption. We evaluated the effects of frost on nutrient loss of mangroves and the protective effects of warmer seawater inundation on reducing nutrient loss. On January 14, 2009, the most cold-tolerant mangrove Kandelia obovata at its naturally latitudinal limit (Fuding, China, 27°17??N) was exposed to freezing temperature (?2.4°C) for 4 h (minimum ?2.8°C). The freezing air temperature occurred during flood tide, resulting that the flooded shoots were protected by warmer seawater. Frost caused 31.3% and 13.0% defoliation on the exposed shoots and the flooded shoots, respectively. Frost restricted nutrient resorption during leaf senescence. K. obovata resorbed 61% N and 42% P during normal leaf senescence, respectively. However, frost-damaged leaves only resorbed 13% N and 10% P during the course, respectively. Foliar N:P molar ratios were <31, suggesting N limitation. Tidal inundation can partially protect mangroves from frost damage. Reduced nutrient resorption efficiency and massive leaf fall caused by frost add pressure to mangroves under nutrient limitation at their latitudinal limits.     

Shoot development of Norway spruce (Picea abies) involves changes in piperidine alkaloids and condensed tannins

Key Message

Secondary chemistry of P. abies peaks early in shoot development. Condensed tannins accumulate already in late buds while piperidine alkaloid biosynthesis take place in early stage shoots.

Abstract

Plants protect new vegetative parts with defensive secondary metabolite compounds. We investigated how concentrations of piperidine alkaloids and condensed tannins change during bud burst and shoot growth in adult Picea abies. We detected 12 individual piperidine compounds, of which epipinidinone and 1,2-dehydropinidinone and two tentatively identified 1,6-imines are reported for the first time in P. abies. In addition three piperidine alkaloid compounds remain partly identified. We found that concentrations of both total piperidine alkaloids and condensed tannins were highest immediately after bud burst. While concentrations of condensed tannins started to increase during bud opening, the dilution effect decreased concentrations in the developing needles of mature branches. By contrast, the decrease of total alkaloid concentrations in mature shoots was not due to the dilution effect, but was connected to the disappearance of precursor components of biosynthesis. The concentrations of major alkaloid components remain stable from dormant buds to mature needles and twigs, underlining their importance for P. abies, although their real ecological significance is yet to be solved. Based on the structural features and timing of appearance of individual compounds, we also propose a hypothetical biosynthesis route for trans-substituted coniferous piperidine alkaloids.     

Freezing cytorrhysis and critical temperature thresholds for photosystem II in the peat moss Sphagnum capillifolium

Leaflets of Sphagnum capillifolium were exposed to temperatures from ?5°C to +60°C under controlled conditions while mounted on a microscope stage. The resultant cytological response to these temperature treatments was successfully monitored using a light and fluorescence microscope. In addition to the observable cytological changes during freezing cytorrhysis and heat exposure on the leaflets, the concomitant critical temperature thresholds for inactivation of photosystem II (PS II) were studied using a micro fibre optic and a chlorophyll fluorometer mounted to the microscope stage. Chlorophyllous cells of S. capillifolium showed extended freezing cytorrhysis immediately after ice nucleation at ?1.1°C in the water in which the leaflets were submersed during the measurement. The occurrence of freezing cytorrhysis, which was visually manifested by cell shrinkage, was highly dynamic and was completed within 2 s. A total reduction of the mean projected diameter of the chloroplast containing area during freezing cytorrhysis from 8.9 to 3.8 μm indicates a cell volume reduction of approximately ?82%. Simultaneous measurement of chlorophyll fluorescence of PS II was possible even through the frozen water in which the leaf samples were submersed. Freezing cytorrhysis was accompanied by a sudden rise of basic chlorophyll fluorescence. The critical freezing temperature threshold of PS II was identical to the ice nucleation temperature (?1.1°C). This is significantly above the temperature threshold at which frost damage to S. capillifolium leaflets occurs (?16.1°C; LT₅₀) which is higher than observed in most higher plants from the European Alps during summer. High temperature thresholds of PS II were 44.5°C which is significantly below the heat tolerance of chlorophyllous cells (49.9°C; LT₅₀). It is demonstrated that light and fluorescence microscopic techniques combined with simultaneous chlorophyll fluorescence measurements may act as a useful tool to study heat, low temperature, and ice-encasement effects on the cellular structure and primary photosynthetic processes of intact leaf tissues.     

Effects of temperature and irradiance on quantum yield of PSII photochemistry and xanthophyll cycle in a tropical and a temperate species

The effect of a wide range of temperatures (?15 and 60°C) in darkness or under strong irradiation [1,600 μmol(photon) m^?2 s^?1] on quantum yield of photosystem II photochemistry and xanthophyll cycle pigments was investigated in a tropical fruit crop (Musa sp.) and a temperate spring flowering plant (Allium ursinum L.). In darkness within the nonlethal thermal window of A. ursinum (from ?6.7 to 47.7°C; 54.5 K) and of Musa sp. (from ?2.2°C to 49.5°C; 51.7 K) maximal quantum yield of PSII photochemistry (F_v/F_m) was fairly unaffected by temperature over more than 40 K. At low temperature F_v/F_m started to drop with ice nucleation but significantly only with initial frost injuries (temperature at 10% frost damage; LT₁₀). The critical high temperature threshold for PSII (T_c) was 43.8°C in A. ursinum and 44.7°C in Musa sp. Under strong irradiation, exposure to temperatures exceeding the growth ones but being still nonlethal caused photoinhibition in both species. Severity of photoinhibition increased with increasing distance to the growth temperature range. ΔF/F_m′ revealed distinctly different optimum temperature ranges: 27–36°C for Musa sp. and 18–27°C for A. ursinum exceeding maximum growth temperature by 2–7 K. In both species only at temperatures > 30°C zeaxanthin increased and violaxanthin decreased significantly. At nonlethal low temperature relative amounts of xanthophylls remained unchanged. At temperatures > 40°C β-carotene increased significantly in both species. In Musa sp. lutein and neoxanthin were significantly increased at 45°C, in A. ursinum lutein remained unchanged, neoxanthin levels decreased in the supraoptimal temperature range. In darkness, F_v/F_m was highly temperature-insensitive in both species. Under strong irradiation, whenever growth temperature was exceeded, photoinhibition occurred with xanthophylls being changed only under supraoptimal temperature conditions as an antiradical defence mechanism.     

On the mechanisms of frost injury and frost hardening of spruce chloroplasts

Hill reaction and noncyclic photophosphorylation of isolated class C chloroplasts of spruce (Picea abies (L.) Karst.), as well as ¹⁴CO₂ fixation by whole needles at constant laboratory conditions proceeded at high rates during spring and early summer, declined during late summer and autumn by about 60%, remained at this level during winter, and recovered quickly in early spring. During summer, the whole needles proved to be frost labile, since after exposure to-20°C and careful thawing, fast chlorophyll degradation occurred. In addition, only photosynthetically inactive chloroplasts could be isolated from those precooled needles. On the contrary, during winter the photochemical activities of plastids from freshly harvested needles did not differ from those of artificially frozen-thawed needles. When isolated spruce chloroplasts were exposed to the same subfreezing temperatures as the whole needles, no influence of freezing on the photochemical activities was observed, irrespective of whether the plastids were isolated from frost sensitive or frost hardened needles. It is concluded that frost damage to spruce chloroplasts is due to an attack of membrane toxic compounds or lytic enzymes which were liberated upon freezing from more labile compartments. Frost hardening of the chloroplasts, as determined by the stability of chlorophyll after exposure of the needles to low temperatures, as well as by the isolation of photosynthetically active chloroplasts from such precooled needles, appeared to depend at least on 2 processes: (i) an alteration of the composition of the photosynthetically active membranes and (ii) and additional stabilization of these membranes by protecting substances. The first process was indicated by a large increase (decrease) of the capability of isolated chloroplasts for PMS-mediated photophosphorylation which accompanied natural or artificial frost hardening (dehardening). Production of cryoprotecting compounds was suggested by a significant higher stability against NaCl observed with class C chloroplasts isolated from frost hardened needles as compared to that of plastids from frost labile material. The decrease of the capability for both, the ferricyanide dependent photoreactions of the plastids and the CO₂ fixation by whole needles, which was observed during the frost hardening phase, cannot be due to freezing injuries; it rather appears to be a consequence of the frost hardening process.     

Characterization of phytohormonal and postharvest senescence responses of balsam fir (Abies balsamea (L.) Mill.) exposed to short-term low temperature

To fulfill the US Thanksgiving and Christmas tree markets, balsam fir (Abies balsamea (L.) Mill.) is generally harvested before the cold season, anecdotally leading to premature needle senescence. Accordingly, we tested the hypothesis that LT exposure before harvest induces specific hormonal changes and delays postharvest senescence and/or abscission in balsam fir. Two hundred and six seedlings exposed to two temperature treatments for 48?h, LT at 5?°C and controls at 22?°C were severed off roots and monitored for their postharvest needle senescence. Root and shoot (needles and buds) tissues were examined for major endogenous hormone metabolites. LT increased shoot ABA (2,007?ng?g^?1 DW) by 2.5× and decreased GA₄₄ (9.84?ng?g^?1 DW) by 3.5× over those in roots. LT did not alter cytokinins, auxins or any root hormonal concentration. With auxins, only IAA, IAA-Asp, IAA-Leu and IAA-Glu were detected and the concentrations of IAA and IAA-Asp in shoots were lower than those found in roots. Among cytokinins, shoot c-ZR (58.95?ng?g^?1 DW) and t-ZR (4.17?ng?g^?1 DW) were 3× higher than those in roots. Apart from GA₄₄, GA₉ (136.76?ng?g^?1 DW) was abundant in shoots. The PBL and PNL were 46 and 1.2?%, irrespective of treatments. LT seedlings held needles 11?days longer than the controls (122?days). In balsam fir, short-term LT exposure augmented ABA and decreased GA₄₄ levels in shoots and delayed postharvest needle senescence.